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Trimmer capacitors provide the means of adjusting a circuit's capacitance once all components are assembled onto a printed-circuit board (PCB). For the purpose of experimentation, or even for tuning a circuit's performance on the production line, a well-chosen trimmer capacitor can provide the necessary capacitance range to adjust the center frequency of filters, oscillators, and other high-frequency components. And while adding a trimmer once meant adding size to a circuit, Voltronics Corp. (www.voltronicscorp.com) now offers its J series of ceramic chip-sized trimmer capacitors that require no compromise of size for flexibility. The tiny trimmer capacitors include the models JB, JN, JQ, JR, JT, JV, and JZ products (see figure). They offer individual capacitance ranges as wide as 10 to 120 pF and support self-resonant frequencies as high as 6 GHz.

The trimmers feature an elegant architecture for such simple circuit elements, with non-rotating pistons. As a result, the position and height of the tuning screw remains fixed on the trimmer capacitor body. Also, the J series trimmer capacitors include an O-ring seal for protection against contamination; the seal maintains its integrity even as the capacitance value is adjusted. They are suitable for a wide range of surface- mount and printed-circuit-board (PCB) applications, with the product series providing various tradeoffs in package size and tuning range. The JB trimmers, for example, measure 0.236 x 0.236 x 0.189 in. (6 x 6 x 4.8 mm), but include models JB900 and JB1200 with among the widest capacitance tuning ranges, at 9 to 90 pF and 10 to 120 pF, respectively. The self-resonant frequencies (SRFs) at the highest capacitance setting for the models JB900 and JB1200 are 200 and 170 MHz, respectively.

When smaller size is more important than capacitance tuning range, the JN series trimmer capacitors measure just 0.067 x 0.059 x 0.035 in (1.7 x 1.5 x 0.9 mm) with a capacitance tuning ranges of 1.5 to 4.0 pF for the model JN040 and from 3 to 8 pF for the model JN080. The SRFs at the maximum capacitance settings for each are 2.7 and 1.8 GHz, respectively. The JN series trimmers are rated for DC working voltage of 25 V and DC withstanding voltage of 55 V. For both trimmer models, the minimum quality factor (Q) at 1 MHz is 300.

When mounting is a critical issue, the the JZ and JR series trimmers have no exposed metal mechanism underneath the ceramic chip packaging. The JZ trimmers measure 0.177 x 0.126 x 0.059 in. (4.5 x 3.2 x 1.5 mm) with typical capacitance ranges of 1.5 to 3.0 pF, 2 to 6 pF, 5.5 to 30 pF, and 8 to 40 pF. For all JZ models, the DC working voltage is rated at 100 V and the DC withstanding voltage is rated at 220 V. The JR series ceramic chip trimmers provide similar ranges and performance, but in a package measuring 0.138 x 0.122 x 0.045 in. (3.5 x 3.1 x 1.15 mm).